1. Field of the Invention
The present invention relates to plasma discharge devices, such as for generating ozone, for example; and more particularly to the high voltage power supply for such plasma discharge devices.
2. Description of the Related Art
High energy plasmas are used for a variety of purposes, such as ionizing gas for the generation of ozone or to reduce undesirable nitrogen oxide automobile emissions. FIG. 1 shows a block diagram of a conventional apparatus for generating ozone and is typical of most equipment for generating a plasma with different types of gases. The high volume plasma generator 10 comprises a plurality of plasma discharge cells 12, 13, and 14 each having the schematic design shown for the first cell 12. The plasma discharge cell includes a chamber 16 containing the gas that is to be excited to produce the plasma. The chamber may be closed or, as is the case for an ozone generator, may have a passageway into which oxygen enters and the generated ozone exits. A pair of electrodes 17 and 18 are spaced apart on opposite sides of the chamber 16. When a high voltage is applied across the electrodes, the gas within the chamber 16 is excited, thereby producing the plasma that coverts the incoming oxygen (O2) into ozone (O3). Each plasma discharge cell exhibits a large capacitance load.
The plasma discharge cells 12-14 are driven by a power supply which receives alternating electric current at an input to an inverter 20. The inverter 20 converts the line frequency of the input electric current to a higher frequency suitable for exciting the gas of interest. The output of the inverter 20 is coupled by an inductor/choke 22 to a set of high voltage transformers 24, 25, and 26 connected in parallel. Each transformer 24, 25, and is associated with a different one of the plasma discharge cells 12, 13, and 14, respectively.
The capacitive load of each plasma discharge cell 12-14 is reflected through the respective high voltage transformer 24-26 and the choke 22 to the electronics of the inverter 20. That capacitive load can vary dynamically due to manufacturing tolerances of the plasma generator, as well as variation of the pressure, temperature, and flow rate of the gas being excited. The combination of that capacitive load along with the inductance and resistance of the associated power supply branch form a separate series resonant circuit for each plasma discharge cell. Although those resonant circuits have identical designs to theoretically resonant at the same frequency, the manufacturing tolerances and dynamic gas parameter variations cause each circuit branch to have a different resonant frequency. Nevertheless a single inverter 20 is employed to simplify tuning of the resonance and to eliminate beat frequencies that would exist if multiple inverters were employed in the same plasma generator.
A disadvantage with such conventional power supplies for multiple plasma discharge cells is the relatively large size of the magnetic components, i.e. the choke 22 and transformers 24-26, which significantly add to the cost and weight of the apparatus.
Furthermore, conventional design practice dictates that each transformer for a multiple cell plasma generator be constructed so that its primary and secondary coils are tightly coupled magnetically to reduce stray magnetic fields by minimizing the internal flux leakage. The sum of the transformer leakage inductance and the external choke inductance creates an aggregate inductance that ultimately balances the capacitance of the associated plasma discharge cell. In other words, each transformer has a core that maximizes the conductance of magnetic flux between the primary and secondary coils.
Furthermore, standard engineering practice is to physically separate the transformers 24-26 and the choke 22 by an amount that minimizes the stray magnetic field coupling between those components and to the enclosure of the power supply. Metal objects within such stray magnetic fields become heated to undesirable temperatures. However, separating the magnetic components from each other and from other metal objects within the apparatus has the drawback of requiring a significant amount of empty space within the device. Therefore, conventional design practice dictates that it is desirable to tightly couple the primary and secondary coils of each transformer so as to minimize the stray fields originating from the component.